Electromagnetic apparatus drive apparatus

ABSTRACT

Conventionally, a non-conductive period is provided in a region in the vicinity of zero of an AC power voltage via a voltage detection circuit  14  to turn off reliably. The FET  17  maintains the ON state within several switching cycles after the non-conductive interval to rapidly restore the magnetizing coil current, so that the magnetizing coil current rapidly increases. An object is to suppress beat noise in the electromagnetic device. Within a prescribed interval following the non-conductive interval, a partial voltage at a resistor  19  of an output V 2  of a mono-stable circuit  20  is added as a bias voltage to a detection voltage of a magnetizing coil current at a resistor  18,  and is detected by the IC  11.  The IC  11  drives a FET  17  with a constant switching period after the non-conductive interval, thereby preventing the increase in the magnetizing coil current and resolving the problem.

TECHNICAL FIELD

The present invention relates to a drive unit for an electromagneticdevice, in which a drive current for energizing a magnetizing coil of anelectromagnetic device is controlled with constant-current controlthrough switching means for switching power source to reduce powerconsumption of the electromagnetic device. In particular, the presentinvention relates to a drive unit for an electromagnetic device in whichnoise generated from the electromagnetic device due to an operation ofthe switching means is reduced.

BACKGROUND OF THE INVENTION

Switching means switches an electric current supplied to a magnetizingcoil of an electromagnetic device to reduce power consumption of theelectromagnetic device as disclosed in Japanese Patent No. 2626147. Inthe disclosed technology, a switching control circuit drives powerdistribution to a magnetizing coil of an electromagnetic deviceaccording to an intermittent pulse signal. A main switching element of acontact-less relay inserted between the magnetizing coil of theelectromagnetic device and an AC power source is switched to close andrelease the electromagnetic device. The main switching element in thecontact-less relay becomes a non-conductive state in a region in thevicinity of zero of the power source voltage below a self-holdingcurrent for a predetermined period of time longer than a cycle of theintermittent pulse signal output from the switching control circuit.Accordingly, even if an OFF command is sent to the contact-less relay,an AC path of the contact-less relay maintains a conductive state, sothat the electromagnetic device can be released.

FIG. 4 is a view showing a circuit diagram of a conventional drive unitfor an electromagnetic device in which power consumption of theelectromagnetic device is further reduced through constant-currentcontrol of a magnetizing current of the electromagnetic device, similarto the technology described above. FIG. 5 is a view showing a basicinner structure of a current mode PWM control IC 11 shown in FIG. 4.FIG. 9 shows operational waveforms of main components shown in FIG. 4,and FIG. 10 shows an operational waveform of a voltage detection circuit14 shown in FIG. 4.

In FIG. 4, reference numeral 4 denotes a magnetizing coil (MC) of anelectromagnetic device such as an electromagnetic contactor connected toa DC output side of a diode bridge 2, and reference numeral 1 denotes acontact-less relay for switching the AC power source to the diode bridge(SSR; Solid State Relay) In this circuit diagram, the contact-less relay1 is switched to close and release the electromagnetic device. Inputterminals T1 and T2 are connected to an AC power source. Outputterminals T3 and T4 of the contact-less relay 1 are connected in seriesto the input terminals T1 and T2. A DC power source E is connected tothe input terminals T5 and T6 of the contact-less relay 1 via a switchSW0 and a light-emitting diode PD of a phototriac coupler PC.

A main triac TR is connected parallel to the phototriac PTr of thephototriac coupler PC, and a resistor R11 is connected between a gate ofthe main triac TR and one terminal thereof. A snubber circuit formed ofa capacitor C10 and a resistor R10 is connected in parallel to the maintriac TR. The diode bridge 2 is connected between the output terminal T2of the contact-less relay 1 and the input terminal T2 of the AC powersource. A series circuit formed of the magnetizing coil (MC) of theelectromagnetic device, a power MOSFET 17 as a main switching elementfor controlling a current Imc of the magnetizing coil 4, and a currentdetection resistor 18 (resistance value of R18) inserted into a sourceside of the MOSFET 17 for detecting the current Imc of the magnetizingcoil 4 is connected to a DC output terminal of the diode bridge 2. Acapacitor 3 is connected in parallel to the series circuit, and aflywheel diode 5 is connected in parallel to the magnetizing coil 4.

A series circuit formed of a resistor 6 and a Zener diode 9 is connectedto the DC output terminal of the diode bridge 2, and a series circuitformed of a resistor 7, a transistor 8 with a base connected to acontact point between the resistor 6 and Zener diode 9, and a capacitor10 is also connected to the DC output terminal of the diode bridge 2.The circuits constitute a power source circuit for generating a constantvoltage supplied to a power source terminal VIN of the current mode PWMcontrol IC 11. PWM stands for Pulse Width Modulation.

A series circuit formed of voltage-dividing resistors 12 and 13 isconnected to the DC output terminal of the diode bridge 2. A voltage 14aat a contact point between the resistors 12 and 13 is inputted into avoltage detection circuit 14 for detecting that a voltage of the ACpower source reaches the vicinity of zero. A voltage between the DCoutput terminals of the diode bridge 2 appears as a double rectifiedvoltage of the AC power source, and is divided with the voltage-dividingresistors 12 and 13 to obtain the voltage 14 a. As shown in FIG. 10, thevoltage detection circuit 14 outputs a voltage V1 at a H level at aninterval t1 when the voltage 14 a becomes below a predetermined lowvoltage detection level VL0, and outputs the voltage V1 at a L leveloutside the interval t1 to be supplied to a feedback input terminal FBof the current mode PWM control IC 11.

The low voltage detection level VL0 is set such that the interval t1becomes longer than an output cycle T of the PWM pulse Vout (describedlater). The capacitor C3 provided between the DC output terminals of thediode bridge 2 serves as a power source with respect to a high-frequencycomponent in the load current on the DC side of the diode bridge 2. Dueto a small capacitance of the capacitor, a voltage waveform between theDC output terminals of the diode bridge 2 becomes double rectifiedvoltage waveform following a change in the AC power source voltage.

A PWM control pulse (PWM pulse) Vout is outputted from the OUT terminalof the current mode PWM control IC 11, and is inputted into the gate ofthe power MOSFET 17. A current detection voltage Vcs (=(resistance valueR18 of the resistor 18)×(current Imc of the magnetizing coil 4)) isgenerated at both ends of the current detection resistor 18, and isinputted into the current detection terminal CS of the current mode PWMcontrol IC 11 via the resistor 19.

Reference numerals 15 and 16 denote a timing resistor and a timingcapacitor for determining the cycle of the PWM pulse of the current modePWM control IC 11. The timing resistor 15 is connected between an outputterminal Vref of the IC 11 having a reference voltage (in the presentexample, 5 V) and a timing resistance/capacitance connection terminalRT/CT. The timing capacitor 16 is connected between the terminal RT/CTof the IC 11 and a negative-side terminal of the diode bridge 2. Aground terminal GND of the IC 11 (see FIG. 5) is connected to thenegative-side terminal of the diode bridge 2.

In this case, the current mode PWM control IC for switching the powersource performs the constant voltage control of the switching powdersource voltage, while controlling the load current thereof, is used asthe current mode PWM control IC 11. In the present example, the ICperforms the constant current control when the load of the switchingpower source becomes large, more specifically, when an error amplifieroutput voltage Vcomp (described later) exceeds a prescribed value.

A function of the current mode PWM control IC 11 related to the constantcurrent control will be explained next with reference to FIGS. 4, 5 and9. As shown in FIG. 5, when a voltage supplied to the power sourceterminal VIN of IC 11 becomes a normal operation mode voltage (in thepresent example, 16 V) of the IC 11, a lock of the low-voltage lock-outcircuit UVL1 is released to turn on a 5 V band gap reference voltageregulator REG. Accordingly, the reference voltage Vref of 5 V isgenerated from the voltage supplied to the power source terminal VIN,and is outputted to the terminal Vref of the IC 11 and other componentslocated in the IC 11 as necessary.

When the regulator REG outputs the reference voltage Vref greater than4.7 V, a lock of another low-voltage lock-out circuit UVL2 is released.Also, an output of an OR circuit G2, i.e. one of inputs of a NOR circuitG1, becomes “L”, thereby releasing one of conditions for stopping anoutput of the PWM pulses Vout from a totem pole output circuit TTPdriven by an NOR circuit G1. Conversely, before the release, at leastthe output of the PWM pulse Vout is stopped and the power MOSFET 17using the PWM pulse Vout as a gate input is maintained in an OFF state.

An oscillator OSC generates a triangular wave W1 for determining anoutput cycle T of the PWM pulse Vout. That is, when an output of acomparator CP1 constituting the oscillator OSC is “L”, semiconductorswitches SW1 and SW2 also constituting the oscillator OSC are OFF, and avoltage of 2.8 V as an upper limit voltage of the triangular wave W1 isinputted in an (−) input terminal of the comparator CP1. The timingcapacitor 16 is charged with the reference voltage Vref via the timingresistor 15. The charge voltage of the timing capacitor 16 is inputtedinto an (+) input terminal of the comparator CP1 via the timingresistance/capacitance connection terminal RT/CT of the IC 11.

When the charge voltage of the timing capacitor 16 is about to exceed2.8 V, the output of the comparator CP1 is changed to “H”. As a result,the semiconductor switches SW1 and SW2 are turned ON, and the voltage ofthe (−) input terminal of the comparator CP1 is switched to 1.2 V, i.e.a lower limit voltage of the triangular wave W1. Also, the constantcurrent source IS1 is connected to the terminal RT/CT of the IC 11, andthe timing capacitor 16 starts discharging.

When the voltage of the timing capacitor 16 is about to become below 1.2V, the output of the comparator CP 1 is changed to “L”, and the voltageof the timing capacitor 16 increases, thereby generating the continuoustriangular wave W1.

At this time, the comparator CP 1 outputs an oscillation output W2composed of a square pulse. The oscillation output W2 is inputted into alatch set pulse generation circuit LS. The pulse generation circuit LSgenerates a latch set pulse P1 each time the oscillation output W2rises, and supplies the pulse to a NOR circuit G1 and a set inputterminal S of a current detection latch FF composed of an RS flip-flop.

When the latch set pulse P1 is inputted, an inverted output QB (Bstanding for bar) of the current detection latch FF becomes “L” and atotal input of the NOR circuit G1 becomes “L”. Accordingly, an output ofthe totem pole output circuit TTP, i.e. the PWM pulse Vout outputtedfrom the OUT terminal of the IC 11, becomes the H level to turn on theexternal power MOSFET 17. The PWM pulse Vout maintains at the H level,i.e. the power MOSFET 17 turned on, until the current detection latch FFis reset and the inverted output QB thereof becomes “H”. A reset signalto the input terminal resistor of the current detection latch FF issupplied as the output of the CS comparator CP2. The output of thecomparator CP2 is generated when the power MOSFET 17 is turned on andthe voltage Vcs of the current detection terminal CS, i.e. the voltageof the (+) input terminal of the CS comparator CP2, gradually increasesand exceeds the voltage Vcsn at the (−) input terminal of the CScomparator CP2.

As shown in FIG. 4, in the voltage detection circuit 14, the voltage V1applied to the feedback input terminal FB of the IC 11 only at theinterval t1 in the vicinity of the zero of the AC power source voltage,i.e. the voltage of (−) input terminal of the error amplifier EA, is theH level, and is the L level at an outside of the interval t1. In thepresent example, the H level of the voltage V1 is higher than thevoltage (2.5 V) of the (+) input terminal of the error amplifier EA, andthe L level of voltage V1 is almost 0 V.

Therefore, at the interval t1, an output voltage (error voltage) Vcompof an error amplifier EA is at least 1.4 V or less, and the (−) inputterminal voltage Vcsn of the CS comparator is almost 0 V. At an outsideof the interval t1, the error voltage Vcomp is at least 4.4 V or more,and the (−) input terminal voltage Vcsn of the CS comparator is fixed to1 V of the Zener voltage as the upper limit value. Accordingly, at anoutside of the interval t1, the magnetizing coil current Imc increasesafter the power MOSFET 17 is turned on. As a result, the voltage of thecurrent detection resistor 18, i.e. the voltage (“CS terminal voltage”)Vcs of the current detection terminal CS of the IC 11, graduallyincreases and reaches 1 V of the (−) input terminal voltage Vcsn of theCS comparator, so that the CS comparator CP2 executes an operation ofresetting the current detection latch FF.

A time interval from setting to resetting of the current detection latchFF corresponds to a pulse width (interval of H level) of the PWM pulseVout, i.e. an ON interval of the power MOSFET 17. The time intervalbecomes longer when the current Imc of the magnetizing coil 4 at aninitial stage of the ON interval is small, and becomes shorter as themagnetizing coil current Imc increases and approaches the set value(corresponding to 1 V of the (−) input terminal voltage Vcsn of the CScomparator). The constant current control by the PWM control of thecurrent Imc of the magnetizing coil 4 is performed as described above.

On the other hand, at the interval t1, the (−) input terminal voltageVcsn of the CS comparator becomes zero. Therefore, the pulse width ofthe PWM pulse Vout, i.e. the ON interval of the power MOSFET 17, becomes0 due to the operations shown in FIG. 5. In an actual case, the pulsewidth enters a non-sensitivity zone, so that the PWM pulse Vout is notoutputted and the power MOSFET 17 remains off.

An operation of the entire configuration shown in FIG. 4 will beexplained with reference to FIG. 9. When the AC power source isconnected to the input terminals T1 and T2 of the AC power source and aswitch SW0 provided between the input terminals T5 and T6 of thecontact-less relay 1 is turned on, the phototriac coupler PC of thecontact-less relay 1 is turned on. As a result, a current flows to thegate of the main triac TR to turn on the main triac TR, and an AC inputvoltage is applied to the diode bridge 2. The capacitor 10 is chargedvia the transistor 8 until the voltage fully rectified by the diodebridge 2 exceeds the Zener voltage of the Zener diode 9. When the fullyrectified voltage of the diode bridge 2 exceeds the Zener voltage of theZener diode 9, the capacitor 10 accumulates an electric chargecorresponding to the Zener voltage of the Zener, thereby obtaining theconstant voltage.

The voltage of the capacitor 10 is inputted to the power source terminalVIN of the current mode PWM control IC 11 to start a normal operation ofthe IC 11. During the time when the output voltage V1 of the voltagedetection circuit 14, i.e. the voltage of the feedback input terminal FBof the IC 11, is at the L level, the current Imc of the magnetizing coil4 is controlled with the constant current control through the switchingin the PWM control of the power MOSFET 17 according to the operation ofthe IC 11 described above.

That is, the PWM pulse Vout of the H level is outputted and the powerMOSFET 17 is switched on for each period T in which the latch set pulseP1 in the IC 11 is outputted. Accordingly, the fully rectified voltageof the diode bridge is applied to the magnetizing coil 4 via the currentdetection resistor 18, and the current Imc of the magnetizing coil 4increases. At this time, a slope of the magnetizing coil current Imc ismainly determined by an inductance of the magnetizing coil 4 and aninstantaneous value of the fully rectified voltage. When the voltage(R18×Imc) of the current detection resistor 18, i.e. the CS terminalvoltage Vcs of the IC 11, reaches 1 V of the (−) input terminal voltageVcsn of the CS comparator of the IC 11 with the increase in themagnetizing coil current Imc, the PWM pulse Vout becomes the L level.Also, the power MOSFET 17 is turned off, and the current Imc of themagnetizing coil 4 flows to the flywheel diode 5, and is attenuatedwhile circulating in the magnetizing coil 4 and diode 5. A time constantof the current attenuation is determined by an impedance of themagnetizing coil 4 and a resistance of the circulation flow path.

When the power MOSFET 17 is turned on, the magnetizing coil current Imcis again switched to rising. In such an operation, immediately after theswitch SW0 of the contact-less relay 1 is turned on, the magnetizingcoil current Imc is not established within one output cycle T of thelatch set pulse P1. Accordingly, the voltage of the current detectionresistor 18, i.e. the CS terminal voltage Vcs of the IC 11, does notreach 1 V. As a result, as shown by an enlarged portion of time axis inFIG. 9, the current detection latch FF in the IC 11 is not reset, andthe power MOSFET 17 substantially maintains the ON state.

The magnetizing coil current Imc is established and the CS terminalvoltage Vcs reaches 1 V after several output cycles T of the latch setpulse P1 pass (point of time τc shown in FIG. 9). Then, the ON/OFFoperation of the power MOSFET 17 per each period T is executed and themagnetizing coil current Imc is maintained at an almost constant value,thereby reducing power consumption in the magnetizing coil 4.Accordingly, when the magnetizing coil current Imc is established, theelectromagnetic device, i.e. the electromagnetic switch in the presentexample, is closed.

In the interval t1 where the AC power source voltage is close to zero,the power MOSFET 17 is held in the OFF state as described above. Theinterval t1 is selected to be larger than the ON/OFF period T of thepower MOSFET 17 and the turn-off time interval of the main triac TR ofthe contact-less relay 1. If the input switch SW0 of the contact-lessrelay 1 remains closed, the attenuation of the magnetizing coil currentImc within the interval t1 is comparatively large, as shown in FIG. 9.The main triac TR of the contact-less relay 1 is conductive again afterthe interval t1, so that the ON/OFF operation of the power MOSFET 17 pereach period T is performed via the ON interval tr of the power MOSFET 17containing several periods T.

On the other hand, when the input switch SW0 of the contact less relay 1is opened, the main triac TR of the contact-less relay 1 is turned offwithin the first interval t1 after the opening. The rectified outputvoltage of the diode bridge 2 disappears, and the current Imc of themagnetizing coil 4 is attenuated while being commuted to the flywheeldiode 5, and disappears. The release of the electromagnetic device iscarried out during this attenuation.

At the initial point of time of the electromagnetic device closing andin the holding interval of the electromagnetic device after closing, theconfiguration actually allows the value of the current detectionresistor 18 to be changed with means which is not shown in the figure.In the holding interval of the electromagnetic device, the magnetizingcoil current Imc is made smaller than that at the initial point of timeof closing, thereby reducing power consumption. The waveform in FIG. 9shows an example at the holding time of the electromagnetic device.

Strictly speaking, in a section indicated by a projected line in theenlarged portion of time axis (interval tr) of the CS terminal voltageVcs shown in FIG. 9, i.e. a very small interval in which the latch setpulse P1 is present, the output of the NOR circuit G1 in the IC 11becomes “L” and the PWM pulse Vout is at the L level. The power MOSFET17 is instantaneously driven OFF, and is maintained in the ON state dueto a turn-off delay of the power MOSFET 17.

The device shown in FIG. 4 has the following problems. That is, as shownin FIG. 9, within the holding interval of the electromagnetic device,when the main triac TR of the contact-less relay 1 is transited from thenon-conductive interval to the conductive interval as the interval t1sandwiching the zero cross point of the AC power source voltage, thecurrent Imc of the magnetizing coil 4 becomes substantially lower thanthe set value in the non-conductive interval t1. Accordingly, thecurrent mode PWM control IC 11 outputs the PWM pulse Vout in asubstantially ON mode within the interval tr significantly longer thanthe usual switching period T. When the magnetizing coil current Imcreaches the set current (holding current of the electromagnetic device),that is, when the CS terminal voltage Vcs reaches 1 V of the (−) inputterminal voltage Vcsn of the CS converter, the PWM pulse Vout is turnedoff.

A variation in the magnetizing coil current Imc in the interval tr (alsoreferred to herein below as the continuous ON interval of the PWM pulseVout or power MOSFET 17) is greater by about an order of magnitude thanthe variation in the current of the current pulsation componentstabilized after the interval. As a result, the attraction force of theelectromagnetic device is greatly fluctuated, thereby causing beat soundfrom the electromagnetic device.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a drive unit for anelectromagnetic device capable of reliably releasing an electromagneticdevice with a non-conductive interval t1. It is possible to reduce powerconsumption through constant current control conducted with PWM controlof a magnetizing coil current of the electromagnetic device, and also toreduce beat noise of the electromagnetic device in a holding state.

In order to solve the problems described above, according to a firstaspect of the present invention, a drive unit of an electromagneticdevice includes a switching control circuit (current mode PWM control IC11) for driving a current to a magnetizing coil of the electromagneticdevice according to an intermittent pulse signal (PWM pulse Vout) viaswitching means (power MOSFET 17). The switching control circuitswitches the pulse signal so that the switching means is switched froman OFF state to an ON state at a first timing of turn-on timingsgenerated in a predetermined period (T). The switching control circuitalso switches the pulse signal so that the switching means is switchedto the OFF state at a timing in which a detected value (CS terminalvoltage Vcs) of the electric current of the magnetizing coil becomes apredetermined value ((−)input terminal voltage Vcsn of the CS comparatorCP2, 1 V in an embodiment). The drive unit closes and releases theelectromagnetic device by switching a main switching element (maintriac) of a contact-less relay (1) inserted between the magnetizing coilof the electromagnetic device and an AC power source. The main switchingelement in the contact-less relay becomes a non-conductive state for apredetermined period of time longer than the predetermined period (viathe voltage detection circuit 14) in a region (interval t1) in thevicinity of zero of a power source voltage below a self-holding current.A predetermined bias signal is superimposed on the current detectionvalue or current set value at least within a predetermined interval (t2)following the time interval of the non-conductive state. The switchingcontrol circuit switches the pulse signal so as to switch the switchingmeans per each predetermined period.

According to a second aspect of the present invention, in the drive unitfor the electromagnetic device in the first aspect, the bias signal is acontinuous signal (divided value (voltage of a resistance 19) of anoutput voltage V2 of a mono-stable circuit) at a predetermined level(via the mono-stable circuit 20 and the like).

According to a third aspect of the present invention, in the drive unitfor the electromagnetic device in the first aspect, the bias signal is asignal (divided value (voltage of the resistance 19) of an outputvoltage V3 of an AND circuit) at a predetermined level present only whenthe switching means becomes the ON state (via the mono-stable circuit20, AND circuit 23, or the like).

According to a fourth aspect of the present invention, in the drive unitfor the electromagnetic device in the third aspect, the pulse signal forcausing the switching means to become the ON state (via the resistor 22or the like) is used for the bias signal.

According to a fifth aspect of the present invention, in the drive unitfor the electromagnetic device in the first aspect, the bias signal is asignal of a predetermined waveform having a level decreasing with time.

An effect of the present invention is as follows. The drive unit closesand releases the electromagnetic device by switching the main switchingelement of the contact-less relay inserted between the AC power sourceand the magnetizing coil of the electromagnetic device controlled withthe constant-current control by switching the switching means (powerMOSFET 17) with the PWM control according to the synchronization signal(latch set pulse P1) in the prescribed period (T). In order to maintainthe main switching element of the contact-less relay in a conductivestate so that the electromagnetic device can be released even if an OFFcommand is supplied to the contact-less relay, the predetermined biassignal is superimposed on the current detection value or current setvalue at least within the predetermined interval (t2) following thenon-conductive interval (t1) provided in the region in the vicinity ofzero of the AC power source voltage. As a result, the switching means,within the period corresponding to the predetermined period (T) in theON state, apparently is switched to the OFF state in which the currentin the magnetizing coil reaches the set value. The switching means isswitched per the predetermined period (T) immediately after anon-conductive period, thereby gradually increasing the current in themagnetizing coil to the set value.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram illustrating a configuration according to afirst embodiment of the present invention;

FIG. 2 is a circuit diagram illustrating a configuration according to asecond embodiment of the present invention;

FIG. 3 is a circuit diagram illustrating a configuration according to athird embodiment of the present invention;

FIG. 4 is a conventional circuit diagram corresponding to FIGS. 1 to 3;

FIG. 5 is a circuit diagram illustrating a configuration of an innerpart of a current mode PWM control IC 11 shown in FIGS. 1 to 4;

FIG. 6 is a waveform diagram illustrating an operation of maincomponents shown in FIG. 1;

FIG. 7 is a waveform diagram illustrating an operation of maincomponents shown in FIG. 2;

FIG. 8 is a waveform diagram illustrating an operation of maincomponents shown in FIG. 3;

FIG. 9 is a waveform diagram illustrating an operation of maincomponents shown in FIG. 4; and

FIG. 10 is a waveform diagram for explaining an operation of a voltagedetection circuit 14 shown in FIGS. 1 to 4.

DESCRIPTION OF REFERENCE NUMERALS AND SYMBOLS

-   1: CONTACT-LESS RELAY (SSR), SW0: INPUT SWITCH OF CONTACTLESS RELAY,    PC: PHOTOTRIAC OF CONTACTLESS RELAY, TR: MAIN TRIAC OF CONTACTLESS    RELAY, 2: DIODE BRIDGE, 3: CAPACITOR, 4: MAGNETIZING COIL (MC) OF    ELECTROMAGNETIC DEVICE, Imc: ELECTRIC CURRENT OF MAGNETIZING COIL 4,    5: FLYWHEEL DIODE, 6 and 7: RESISTORS, 8: TRANSISTOR, 9: ZENER    DIODE, 10: CAPACITOR, 11: CURRENT PWM CONTROL IC, 12 and 13:    VOLTAGE-DIVIDING RESISTORS, 14: VOLTAGE DETECTION CIRCUIT, 14: INPUT    VOLTAGE OF VOLTAGE DETECTION CIRCUIT 14, V1: OUTPUT VOLTAGE OF    VOLTAGE DETECTION CIRCUIT 14, 15: TIMING RESISTOR, 16: TIMING    CAPACITOR, 17: POWER MOSFET, 18: CURRENT DETECTION RESISTOR, R18:    RESISTANCE VALUE OF CURRENT DETECTION RESISTOR 18, 19:    VOLTAGE-DIVIDING RESISTOR, 20: MONOSTABLE CIRCUIT,-   V2: OUTPUT VOLTAGE OF MONOSTABLE CIRCUIT, 21 and 22:    VOLTAGE-DIVIDING RESISTORS, 23: AND CIRCUIT, V3: OUTPUT VOLTAGE OF    AND CIRCUIT 23, CS: CURRENT DETECTION TERMINAL CS OF IC 11, Vcs:    NPUT VOLTAGE OF CURRENT DETECTION TERMINAL CS OF IC 11,=((+) INPUT    TERMINAL VOLTAGE OF CS COMPARATOR IN IC 11), FB: FEEDBACK INPUT    TERMINAL OF IC 11, RT/CT: TIMING RESISTANCE/CAPACITANCE CONNECTION    TERMINAL OF IC 11, Vref: REFERENCE VOLTAGE OUTPUT TERMINAL OF IC 11,    VIN: POWER SOURCE TERMINAL OF IC 11, OUT: PWM PULSE OUTPUT TERMINAL    OF IC 11, Vout: PWM PULSE, EA: ERROR AMPLIFIER IN IC 11, Vcomp:    OUTPUT (ERROR VOLTAGE) OF ERROR AMPLIFIER, OSC: OSCILLATOR IN IC 11,    LS: LATCH SET PULSE GENERATION CIRCUIT IN IC 11, P1: LATCH SET    PULSE, CP2: CS COMPARATOR IN IC 11, Vcsn: (−) INPUT TERMINAL VOLTAGE    OF CS COMPARATOR, FF: CURRENT DETECTION LATCH IN IC 11, G1: NOR    CIRCUIT IN IC 11, TTP: TOTEM POOLE OUTPUT CIRCUIT IN IC 11

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS First Embodiment

FIG. 1 shows a circuit diagram of a drive device for an electromagneticdevice according to a first embodiment of the present invention. FIG. 6shows an operational waveform of a main part of the circuit shown inFIG. 1 when the electromagnetic device becomes a hold state. Here, FIG.1 corresponds to FIG. 4, and FIG. 6 corresponds to FIG. 9.

The circuit diagram shown in FIG. 1, in addition to the components shownin FIG. 4, comprises an mono-stable circuit 20 and a resistor 21connected between an output terminal of the mono-stable circuit 20 and acurrent detection terminal CS of a current mode PWM control IC 11. Asshown in FIG. 6, the mono-stable circuit 20 is triggered when a voltageV1 of an H level outputted by a voltage detection circuit 14 decreaseswithin a non-conductive interval t1 centered around a 0 cross point withthe AC powder source voltage. A voltage V2 of the H level is outputtedwithin a period t2 comprising a plurality of periods T of a latch setpulse P1 after the voltage V1 decreases.

The interval t2 following the non-conductive interval t1 is selected tobe larger than a substantially ON interval of a PWM pulse Vout in FIG.9, that is, a continuous ON interval tr of a power MOSFET 17. The outputvoltage V2 of the mono-stable circuit 20 is divided by resistors 21 and19 and a current detection resistor 18. As compared with the circuitdiagram shown in FIG. 4, a divided voltage component of the resistors 19and 18 created by the voltage V2 within the interval t2 is added to avoltage (CS terminal voltage) Vcs to be applied to the current detectionterminal CS of the current mode PWM control IC 11. A value R18 of thecurrent detection resistor 18 is substantially lower than that of theresistor 19, so that the divided voltage component becomes almost thevoltage of the resistor 19.

Therefore, within the interval t2, the CS terminal voltage Vcs, as shownby a hidden line in FIG. 6, becomes a superposition of a voltage(Imc×R18) of the current detection resistor 18 created by a current Imcof the magnetizing coil 4 and a voltage of the resistor 19 composed ofthe divided voltage component of a mono-stable circuit output voltageV2, within the interval of the H level of the PWM pulse Vout, that is,the ON period of the power MOSFET 17.

In the embodiment, even within the interval t2, the CS terminal voltageVcs composed of the superimposed voltage reaches an (−) input terminalvoltage Vcsn (in the present example, 1 V) of a CS comparator CP2located in the IC 11 per each output cycle T of the latch pulse P1.Accordingly, within the interval t2 following the non-conductiveinterval t1, the power MOSFET 17 repeats switching per each output cycleT of the latch pulse P1 and the current Imc of the magnetizing coil 4increases to a set value, while repeating small pulsations. Therefore,the beat noise of the electromagnetic device is reduced.

Second Embodiment

FIG. 2 shows a circuit diagram of a drive device for an electromagneticdevice according to a second embodiment of the present invention. FIG. 7shows an operational waveform of a main part of the circuit shown inFIG. 2 when the electromagnetic device becomes a hold state. Here, FIG.2 corresponds to FIG. 4, and FIG. 7 corresponds to FIG. 9.

The circuit diagram shown in FIG. 2, in addition to the components shownin FIG. 4, comprises a resistor 22 connected between the PWM pulseoutput terminal OUT of the current mode PWM control IC 11 and thecurrent detection terminal CS. In the circuit shown in FIG. 2, each timethe PWM pulse Vout of the H level is outputted, the voltage of the PWMpulse Vout is divided by the resistors 22 and 19 and current detectionresistor 18. Accordingly, in this case, the superimposed voltage of thedivided voltage component of the PWM pulse Vout, i.e. the voltageapplied to the resistor 19, and the voltage (Imc×R18) of the currentdetection resistor 18 created by the current Imc of the magnetizing coil4 becomes the CS terminal voltage Vcs applied to the current detectionterminal CS of the IC 11.

In the circuit diagram shown in FIG. 2, as shown in FIG. 7, within theinterval following the non-conductive interval t1, the CS terminalvoltage Vcs composed of the superimposed voltage reaches 1 V of the (−)input terminal voltage Vcsn of the CS comparator CP2 located in the IC11 per each output cycle T of the latch pulse P1. The current Imc of themagnetizing coil increases to the set value, while repeating smallpulsations.

Third Embodiment

FIG. 3 shows a circuit diagram of a drive device for an electromagneticdevice according to a third embodiment of the present invention. FIG. 8shows an operational waveform of a main part of the circuit diagramshown in FIG. 3 when the electromagnetic device becomes a hold state.Here, FIG. 3 corresponds to FIG. 1, and FIG. 8 corresponds to FIG. 6.

In the circuit diagram shown in FIG. 3, in addition to the componentsshown in FIG. 1, an AND circuit 23 having one input terminal connectedto the output of the mono-stable circuit 20 is inserted between themono-stable circuit 20 and the resistor 21, and the other input terminalof the AND circuit 23 is connected to the PWM pulse output terminal OUTof the current mode PWM control IC 11. In the circuit diagram shown inFIG. 3, as shown in FIG. 8, within the interval t2 in which the outputV2 of the mono-stable circuit 20 becomes the H level, the intervalfollowing the non-conductive interval t1, i.e. the output voltage V3 ofthe AND circuit 23, becomes the H level only when the PWM pulse Vout ofthe H level is outputted. The superimposed voltage of the dividedvoltage component of the resistor 19 created by the output voltage V3and the voltage (Imc×R18) of the current detection resistor 18 createdby the magnetizing coil current Imc becomes almost the CS terminalvoltage Vcs.

Accordingly, as compared with the circuit diagram shown in FIG. 6, asshown in FIG. 8, an operation within the interval in which the PWM pulseVout is at the H level and the power MOSFET 17 is ON, is similar to thatshown in FIG. 6. The CS terminal voltage Vcs disappears within theinterval in which the PWM pulse Vout is at the L level and the powerMOSFET 17 is OFF. As a result, the power MOSFET 17 is thereby preventedfrom being erroneously switched ON by noise or the like within theinterval in which the power MOSFET 17 is OFF.

Further, in the embodiments described above, the positive bias voltageas a voltage of the resistor 19 is superimposed on the voltage of thecurrent detection resistor 18, that is, the detection voltage of theelectric current of the magnetizing coil 4, within at least thepredetermined interval following the non-conductive interval t1; Asimilar effect can be obtained by superimposing a negative bias voltageof the (−) input terminal voltage Vcsn of the CS comparator CP2 locatedin the IC 11, that is, the set value of the current in the magnetizingcoil 4.

Further, the bias voltage may be a voltage with a waveform decreasingwith time, for example, as the voltage of a capacitor discharged via aresistor serving as a load. Such an embodiment is also included in thepresent invention.

In the drive unit for the electromagnetic device, the non-conductiveinterval is provided in a region in the vicinity of zero of an AC powersource voltage in order to reliably turn off the main switching elementof the contact-less relay inserted between the AC power source and themagnetizing coil of the electromagnetic device controlled with theconstant-current control by switching the switching means when theelectromagnetic device needs to be released.

Conventionally, the switching means maintained the ON state withinseveral switching cycles in the interval immediately after thenon-conductive interval, so that the electric current of the magnetizingcoil greatly attenuated from the set value in the non-conductiveinterval is rapidly returned to the set value. The switching meansswitches at the fixed switching cycle after the magnetizing coil currentrapidly increases and reaches the set value, thereby generating beatnoise in the electromagnetic device.

According to the present invention, the predetermined bias signal issuperimposed on the current detection value or current set value atleast within the predetermined interval following the non-conductiveinterval, the switching means is apparently switched to the off stateafter the magnetizing coil current reaches the set value within thepredetermined switching cycle (composed of the fixed cycle) in which theswitching means becomes the ON state. Accordingly, the switching meansswitches at the predetermined switching cycle immediately after theperiod of the non-conductive interval. Therefore, the magnetizing coilcurrent does not increase rapidly immediately after the non-conductiveinterval without a complex control circuit, thereby reducing beat noise.

1. A drive unit for an electromagnetic device, comprising: a switchingcontrol circuit for providing power to a magnetizing coil of theelectromagnetic device with an intermittent pulse signal via switchingmeans, wherein said switching control circuit switches the pulse signalso that the switching means which is in an off state is turned on at aninitial timing in turn-on timings generated with a predetermined cycle,and the switching means which in an on state becomes the off state at atiming when a detected value of a electric current in the magnetizingcoil becomes a predetermined current value, said drive unit closes andreleases the electromagnetic device by switching a main switchingelement of a contact-less relay inserted between the magnetizing coil ofthe electromagnetic device and an AC power source, said main switchingelement in the contact-less relay becomes a non-conductive state for apredetermined time interval longer than the predetermined cycle in aregion in a vicinity of zero of a power source voltage below aself-holding current, a predetermined bias signal is superimposed on thedetected current value or the predetermined current value at leastwithin a predetermined interval following the time interval of thenon-conductive state, and said switching control circuit switches thepulse signal so that the switching means is switched in everypredetermined cycle.
 2. A drive unit for an electromagnetic deviceaccording to claim 1, wherein said bias signal is a continuous signalhaving a predetermined level.
 3. A drive unit for an electromagneticdevice according to claim 1, wherein said bias signal is a signal havinga predetermined level which is present only when the switching meansbecomes the on state.
 4. A drive unit for an electromagnetic deviceaccording to claim 3, wherein said bias signal is the pulse signalcausing the switching means to become the on state.
 5. A drive unit foran electromagnetic device according to claim 1, wherein said bias signalis a signal having a predetermined waveform with a level decreasing withtime.